Digestive System: Gastrointestinal Tract
Time period: days 21–50
The gut tube forms when the yolk sac is pulled into the embryo and pinched off (see Figure 20.2) as the flat germ layers of the early embryo fold laterally and cephalocaudally (head to tail). Consequently, it has an endodermal lining throughout with a minor exception towards the caudal end. Epithelium forms from the endoderm layer and other structures are derived from the mesoderm.
Initially, the tube is closed at both ends, although the middle remains in contact with the yolk sac through the vitelline duct (or stalk) even as the yolk sac shrinks (Figure 33.1).
The cranial end will become the mouth and is sealed by the buccopharyngeal membrane, which will break in the fourth week, opening the gut tube to the amniotic cavity. The caudal end will become the anus and is sealed by the cloacal membrane, which will break during the seventh week.
Buds develop along the length of the tube that will form a variety of gastrointestinal and respiratory structures (see Chapter 34).
The gut is divided into foregut, midgut and hindgut sections by the region of the gut tube that remains linked to the yolk sac and by the anterior branches from the aorta that supply blood to each part (Figure 33.2).
The foregut will develop into the pharynx, oesophagus, stomach and the first two parts of the duodenum to the major duodenal papilla, at which the common bile duct and pancreatic duct enter. The midgut includes the remainder of the duodenum and the small and large intestine through to the proximal two‐thirds of the transverse colon. The hindgut includes the distal third of the transverse colon and the large intestine through to the upper part of the anal canal.
Each division of the gut is supplied by a different artery. The foregut is supplied by branches from the coeliac artery directly from the descending aorta. The midgut receives blood from the superior mesenteric artery and the hindgut from the inferior mesenteric artery (Figure 33.2).
The foregut grows in length with the embryo, and epithelial cells proliferate to fill the lumen. The tube is later recanalised and only becomes a squamous epithelium during the foetal period. Failure of this normal process causes problems of stenosis (narrowing) or atresia (blocked) in the oesophagus or duodenum.
Part of the foregut tube begins to dilate in week 4, the dorsal side growing faster than the ventral side until week 6. This will become the stomach, and the dorsal side becomes the greater cur- vature. The dorsal mesentery (dorsal mesogastrium) will expand significantly to form the greater omentum.
The stomach rotates to bring the left side around to become the ventral surface, explaining why the left vagus nerve innervates the anterior of the stomach (Figure 33.3). This rotation also moves the duodenum into the adult C‐shaped position.
The midgut also lengthens considerably, looping and twisting as it does so, filling the abdominal cavity. At approximately 6 weeks the midgut grows so quickly there is not enough room in the abdomen to contain it, and it herniates into the umbilical cord (Figure 33.4). The midgut also rotates through 270° counterclockwise (if you were to be looking at the abdomen), bringing the developing cae- cum from the inferior abdomen up the left of the developing small intestine to the top of the abdomen, and around to descend to its adult location in the lower right quadrant. The axis of this rotation is the superior mesenteric artery and the rotation is of particular significance when considering the layout of the small and large intestines and accessory organs in adult anatomy.
The midgut re‐enters the abdomen in week 10, and it is thought that growth of the abdomen together with regression of the mesonephric kidney and a reduced rate of liver growth are important factors in this occurring normally.
The last part of the gut tube, the hindgut, ends initially in a simple cavity called the cloaca. The cloaca is also continuous with the allantois, a remnant of the yolk sac that largely regresses but contributes to the superior parts of the bladder in the human embryo.
A wedge of mesoderm, the urorectal septum, moves caudally towards the cloacal membrane as the embryo grows and folds during weeks 4–7 (Figure 33.5). The urorectal septum divides the cloaca into a primitive urogenital sinus anteriorly and an anorectal canal posteriorly. The urogenital sinus will form parts of the bladder and the urogenital tract.
The cloacal membrane ruptures in the seventh week, opening the gut tube to the amniotic cavity. The caudal part of the lining of the anal canal is thus derived from ectoderm and the cephalic part from endoderm. Subsequently, the caudal part of the anal canal receives blood from branches of the internal iliac arteries and the cephalic part receives blood from the artery of the hindgut, the inferior mesenteric artery. Similarly, portosystemic anastomoses also occur here.
Mesenteries of the gut form as a covering of mesenchyme passing over the gut tube from the posterior body wall of the embryo when the tube is in close contact with it. With growth the gut tube moves further into the abdominal cavity and away from the posterior wall. A bridging connective tissue forms suspending the gut and its asso- ciated organs within the abdomen in a dorsal mesentery for most of its length and a ventral mesentery around the lower foregut region. The ventral mesentery is derived from the septum transversum.
The dorsal mesentery will form the mesenteries of the small and large intestines of the adult gastrointestinal tract, and also forms the greater omentum (Figure 33.6). The ventral mesentery will form the lesser omentum between the stomach and the liver, and the falciform ligament between the liver and the anterior abdominal wall.
The extensive lengthening and rotation of the midgut causes the dorsal mesentery to become considerably larger and more convoluted, and its initial simplicity explains the short diagonal attachment of the mesentery of the small intestine to the posterior abdominal wall in the adult. When the hindgut finds its final position in the foetus the mesenteries of the ascending and de colon fuse with the peritoneum of the posterior body wall.